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Knowing the microscopic structure of a muscle helps understand the macroscopic structures we are going to talk about in the following sections. There are 3 types of muscle found in the body: skeletal, cardiac, and smooth. Cardiac muscle is only found in the muscular wall of the heart; while smooth muscle is present in the head and neck surrounding the blood vessels and within the eyes, the focus of this course will only be on skeletal muscles.
One important property that sets skeletal muscle apart from the other two types is that it is voluntary which means that you have conscious control of it. Consequently, every muscle fiber in the body must have a nerve fiber attached to it in order for it to function. Some things we do automatically are skeletal muscle mediated reflexes in the body. For example if someone asks you to take a breath or swallow you can do that because the muscles performing those actions are voluntary. So, while some skeletal muscles seem to act outside of your conscious control, they are actually voluntary muscle, controlled by reflexes in the central nervous system. It also means that any disturbance in the nerve connection to a particular fiber will render it unusable.
A second thing that sets skeletal muscle apart is that the functional unit is not, as in most tissues, a cell, by rather is referred to as a fiber as it is composed of numerous cells that fuse together to form a single membrane bound, multi-nucleated entity during the embryonic stage of development. These fibers run the entire length of a muscle. The center is filled with structures known as a myofibrils composed of repeating units known as sarcomeres. Each sarcomere consists of myosin and actin molecules layered in such a way that so that surrounding every myosin molecule are actin molecules in such a way that each actin molecule is between three myosin molecules and each myosin is surrounded by six actin molecules. The actin molecules are attached to the actin molecules in the adjacent sarcomere at a structure known as the Z.
Under nervous stimulation a series of changes in the cell cause the actin molecules to ratchet towards each other along the myosin molecule. As the actin molecules come closer together the sarcomere shortens and while each shortening is microscopic the huge number of these small movements becomes macroscopic shortening of the muscle. The last important piece of the puzzle is a protein known as titan which is an elastic protein that both limits stretch and keeps all the parts in the proper alignment.
The regular arrangement of molecules in the muscle fiber resulting in dark bands denoting the position of the thicker myosin molecules. This is known as the A band and does not change in size during contraction. There is a lighter band on the borders of the sarcomere where there are only actin molecules known as the I band. As there is more substance in areas where the actin and myosin overlap there is a lighter area in the center of the sarcomere where the actin molecules are not overlapping the myosin ones. This is known as the M line and this area as well as the I band will get smaller as the muscle contracts and larger as it lengthens. It must be noted that muscles only exert force as the sarcomeres shorten. They only return to a longer state by the action of muscles that have the opposite action. For this reason, each joint must have muscles that have opposing actions.
Each fiber has a layer of supporting and nourishing connective tissue surrounding it. This layer is known as the endomysium. Numerous fibers are bundled into groups of fibers known as fascicles. These have a connective tissue sheath known as the perimysium. It is the fascicle organization you see in meat as the “grain” of the muscle. Surrounding the entire belly of the muscle is another layer of connective tissue known as the epimysium. Each layer has fibers that tie it to the larger layer. The epimysium at the ends of the belly are continuous with the tendons that connect the muscle to the areas of origin and insertion.
Each muscle fiber has a single nerve fiber that innervates it but each nerve fiber attaches to a number of muscle fibers. The attachment point of the nerve to the fiber is known as the motor end plate. When a nerve fiber fires every muscle fiber connected to it will contract maximally. This group of fibers connected to the same nerve fiber will contract simultaneously and are known as a motor unit. This means that the degree of movement is related to the number of motor units that are stimulated not a graded response within the fibers.
